1. Field of the Invention
The present invention relates to a vertically aligned liquid crystal display (hereinafter, referred as “VA-LCD”). More particularly, the present invention relates to a VA-LCD in which contrast ratio in the front viewing direction and tilt viewing directions is improved by using a viewing angle compensation film.
2. Description of the Related Art
There are two primary reasons of degradation of a viewing angle of VA-LCDs: first one is the viewing angle dependence of a perpendicular polarizer and second one is the viewing angle dependence of birefringence of a VA-LCD panel.
U.S. Pat. No. 4,889,412 disclosed a VA-LCD using a −C plate compensation film as a viewing angle compensation film. The main function of the −C plate is to compensate the black state of a VA-LCD when a voltage is not applied. However, a VA-LCD with −C plate compensation film has a problem that leakage of light is caused at a tilt angle because the −C plate can not completely compensate the black state.
In order to solve the problem above, U.S. Pat. No. 6,141,075 disclosed a VA-LCD with two compensation films, a −C plate and a +A plate compensation films. The device can compensate the black state on which a voltage is not applied more well compared to the device with the −C plate only, but the minimum contrast at a tilt viewing angle of 70 degrees is 20:1 at most. Accordingly, there is still need to improve contrast at both of the right angle and tilt angles.
Accordingly, the inventors of the present application reached this invention, feeling that a sheet +A plate and a sheet of −C plate have a limit in compensating the viewing angle.